ITU-T is currently revising the G.694.1 recommendation (G.694.1 “Spectral grids for WDM applications: DWDM frequency grid”, ITU-T Recommendation, June 2002) which defines the most common frequency grid, used for channel spacing in DWDM. Study Group 15 of ITU-T is discussing what could be a more effective grid spacing to be adopted in next generation optical networks.
Bandwidth-variable optical cross connects (BV-OXCs) have been recently proposed to improve frequency spectrum utilization in Wavelength Switched Optical Networks (WSONs). Preliminary BV-OXC implementations have been presented to show the operation with different modulation formats and bit rates. BV-OXCs overcome the fixed ITU-T grid spacing by enabling per port allocation of a configurable portion of the frequency spectrum. The evolution towards a flexible grid is likely to be a “next step” in products having a Reconfigurable Optical Add Drop Multiplexer (ROADM) capability.
There are various different points of view about how to implement such flexible grids to enable improved bandwidth efficiency in photonic in general and in WSON in particular. One challenging aspect of such flexible grid technology is how to implement path computation across a WSON mesh operating on a flexible grid WDM infrastructure. The already complex Routing and Wavelength Assignment (RWA) exercise becomes even more difficult when well defined “channels”, operating on a rigid grid with a fixed bit rate and modulation format, are replaced with flexible “frequency slots” operating on a flexible grid with the additional degree of freedom of the “tunable” modulation format in some cases. The RWA may need to evolve towards becoming effectively a Routing and Spectrum Assignment (RSA) computation, designed to efficiently exploit the flexible spectrum resources. However existing RWA solutions are not directly applicable to BV-OXC due to the different bandwidth granularity of the frequency grid and the optical signals. Dynamic RSA schemes have been proposed in the context of optical orthogonal frequency-division multiplexing (OFDM) where the fine granularity of elastic requests is specifically exploited to occupy a freely variable number of adjacent sub-carriers, as shown in: K. Christodoulopous, et al., “Dynamic bandwidth allocation in flexible OFDM-based networks,” in Proc. of OFC/NFOEC, 2011. It is known to make a frequency slot assignment based on first fit technique, meaning a lowest indexed available set of slots is chosen for a new path.